Our overall goal is to investigate a promising new detector system, plastic scintillator, for brachytherapy dosimetry. Solid organic scintillators convert energy absorbed from ionizing radiation fields into blue fluorescent photons. This measurable optical signal is proportional to absorbed dose in the surrounding medium. The practical efficiency of plastic scintillator is nearly 3 times that of ion chamber and 10-20 times more than that of TLD. Unlike TLD and diode detectors, plastic scintillator can be closely matched to the atomic composition of water, nearly eliminating distance-dependent energy-response artifacts. A major advantage of plastic scintillator over currently-available dosimeters, which are limited to point-by-point measurements, is its potential, in the form of thin sheets, to simultaneously measure dose at every point in an entire plane with data acquisition times on the order of a few minutes for low dose-rate sources with energies of 20 KeV-1 MeV. An areal detector system, capable of accurate and efficient measurement of 2-D dose-rate distributions, would make dose measurement, currently limited to the laboratory setting, more accessible to clinical brachytherapy. In addition, progress would be enhanced in emerging research areas, which require extensive characterization of tissue and applicator heterogeneities, such as image-based 3D treatment planning, optimization of HDR brachytherapy and development of low-energy brachytherapy sources. Our proposal is directed towards both investigation of fundamental physical properties of plastic scintillation dosimeters and optimization of detector and image-acquisition system properties to best realize a useful detector system. Linearity of scintillation yield with dose will be studied as a function of source energy, distance, detector dimensions, and detector composition. A single-element point-detectors optically coupled via a polystyrene fibre to a photomultiplier tube, will be used to study these variables using 1-125, Ir-192, Yb-169 and Cs-137 sources. The study will include modification of waveshifting scintillation dyes to maximize locality of the scintillation process as well as study of LET effects and volume- averaging effects. Where supported by the literature, Monte Carlo simulation will serve as the dosimetric 'gold standard' for verifying accuracy of the scintillation dosimeter and will be TLD dosimetry elsewhere. Two-dimensional scintillation images will be amplified by an image-intensifier tube and captured by a digital camera. Our investigation will focus on manipulation of basic scintillator properties and optical image acquisition system parameters to optimize spatial resolution, sensitivity and signal-to-noise ratio. Spatial resolution of the system will be studied and optimized. Finally, utility and accuracy of the 2-D scintillation dosimetry system will be assessed by measuring 3-D dose distributions in a variety of clinically-relevant homogeneous and heterogeneous geometries.